Magnetic resonance tomography (MRT) is an imaging method that enables tomograms of living organisms such as humans to be produced with high resolution. The patient is supported in a homogeneous magnetic field B0. Gradient cores are used to modify the external magnetic field in the FOV (field of view) such that, firstly, a body slice is selected and, secondly, a spatial coding of the generated MR signals is performed.
During the subsequent reconstruction of the MR signals, for example by way of Fourier transformation, an image of the selected slice is produced that is used for medical diagnostics. Generation and detection of the MR signals are performed with the aid of a high-frequency system that includes a transmitting antenna, which irradiates HF excitation pulses into the patient, and a receiving antenna that detects the emitted HF resonance signals and relays them for the purpose of image reconstruction. Depending on the diagnostic task, the contrast of the MR images can be diversely varied by selecting a suitable pulse sequence, such as a spine echo sequence or a gradient echo sequence, and the sequence parameters associated therewith. The MRT images body structures and consequently constitutes a structural imaging method.
Another method for producing tomograms of a patient is positron emission tomography (PET). This nuclear medicine method is used to display the distribution of a radioactively marked substance in the organism. The substance is introduced into the organism and becomes concentrated in specific body tissues in accordance with its biochemical function. The subsequent decay of the radionuclide in this substance in conjunction with positron emission is detected and measured data are supplied that can be reconstructed to form a tomogram that images chiefly biochemical, that is to say physiological, processes.
PET is thus a method for functional imaging. Since the abovementioned processes take place chiefly in specific body compartments such as organs, PET also supplies structural information, but with a substantially lesser resolution than MRT.
Radiopharmaca with radionuclides contained therein which emit positrons upon decay (β+ decay) are suitable for PET. After a short distance of approximately 1 mm, a positron interacts with an electron. In this process, the two particles are annihilated with the production of two photons in the region of gamma radiation, and these move away from one another on a line, that is to say at an angle of 180°.
Use is made for the purpose of detecting the two photons of a PET detector that is also denoted as a PET camera or PET scanner and which surrounds the measurement object. The PET detector includes a large number of detector elements that are arranged about the measurement object, which is to be imaged.
There are various configurations for the arrangement of the detector elements. Most frequently, the detector elements are arranged on a ring that surrounds the measurement object and is completely occupied with detector elements. Such a PET scanner is also denoted as a stationary block ring system.
However, it is also possible to use a smaller number of large-area position sensitive detector elements in a polygonal arrangement. Moreover, it is possible to use a ring that is occupied only partially with detector elements, there being a need in this case for the ring, and thus the detector elements, to be rotated about the measurement object in order to acquire the requisite measured data. Such a system is also denoted as a rotating block ring system. According to a further general refinement, the PET scanner comprises curved continuous panels.
The directions in a detector ring are usually specified as follows: the direction from the center of the detector ring to the circular circumference on which the detector elements are fastened is the radial direction. The direction along the circular circumference is denoted as the transaxial direction. The axis on which the patient couch is arranged is the axial direction, which is also denoted as z-axis in the attached figures.
In order to improve the total detection efficiency, in modern PET scanners the PET detector usually has a length of at least 15 cm in an axial direction. This can be achieved by stacking a number of detector rings directly one after another, or by using two-dimensionally continuous detectors of large axial dimension and having the length specified above. In the case of a number of detector rings, the total number of the detector elements is typically of an order of magnitude of approximately 10 000.
In the basic design, the detector elements situated exactly opposite one another are firstly connected in a state of electronic coincidence, that is to say they register an event only when a photon originating from a positron decay is respectively registered during a very short interval (3-10 ns) at these two detector elements. Such a design would, however, enable the registration only of photons that originate from a radionuclide that is localized exactly at the center of the relevant detector ring inside the measurement object. In order to detect the decay of radionuclides with localization outside the center, that is to say for pictorially acquiring an extended measurement object, each detector element must be connected in a state of electronic coincidence to a fan of detector elements that are situated opposite this detector element on the detector ring. This arrangement, which is also denoted as computed tomography, is illustrated schematically in FIG. 1.
The PET camera CTI-951R-31 (Knoxville, Tenn., USA) from Siemens is specified as an example of a PET detector ring system. This PET scanner includes 16 detector rings following one after another in an axial direction and in each case having a diameter of 102 cm. 512 BGO (bismuth germanium oxide) crystals sensitive to γ quanta are arranged on each detector ring in a fashion corresponding to a total number of detector elements of 8192 BGO crystals for all the detector rings. Each detector element (BGO crystal) has a size of 6.25 mm (transaxial)×6.75 mm (axial)×30 mm (radial).
The PET scanner can, for example, also include four or six flat detectors. The X ring/4R PET camera (Nucline™), which includes four separate rectangular detectors made from NaI(Tl) crystals with a size of 260 mm×246 mm in each case is given as an example of such a PET scanner. This camera is normally used for examining small objects.
In order to carry out the PET method, the radiopharmacon is injected in the patient or administered by inhalation. The patient is positioned on a moveable table such that the body section to be examined is situated in the target region of the detectors. A complete PET scan includes the detection of a large number of photon pairs that is typically in the range from 106 to 108. The subsequent image reconstruction converts the signals thus detected into a 2D image, and this reproduces in a quantitative way the distribution of the radiopharmacon in the measurement object.
As a structural imaging method and a functional imaging method, respectively, MRT and PET supply different information. It is therefore sensible to combine the image information of the two methods, something which enables particularly precisely determined anatomical structures to be assigned to regions with a high concentration of the radiopharmacon, such as organs or cancerous ulcers. The relatively low spatial resolution of PET can be overcome in this way. For future systems, an attempt is therefore currently being made to combine the imaging methods of MRT and PET in one unit, and to render them capable of use as simultaneously as possible.
It is pointed out in this context that combinations of a PET scanner and a computer tomography are already commercially available. For the purpose of examination in such a combined PET/CT unit, the patient is moved on the patient couch directly one after another through the detectors of the two components. Subsequently, the images produced are superimposed in the computer, thus combining the high spatial resolution of a CT with the functional information from PET.
Combined PET/MRT units are already currently under development. Here, an APD photodiode array with an upstream array composed of LSO crystals finds favor as PET detector.
It is to be considered when developing combined PET/MRT units that PET is one of the most expensive imaging methods in medicine. A not insubstantial contribution to the high costs is made by the costly PET detector. Consequently, cost effective approaches to the configuration of the PET detector are advantageous for mass production with reference to a commercial PET/MRT unit.
A combined PET/MRT unit is described in US patent application US 2003/0090267 A1.